Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. In general, the method for producing such glass-ceramics customarily involves three fundamental steps: first, melting a glass-forming batch containing the selected metallic oxides; second, cooling the melt to a temperature at least below its transformation range, while simultaneously forming a glass body of a desired geometry; and third, heating the glass body to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ. To develop nuclei in the glass, the glass will be heated initially to a temperature within or somewhat above the transformation range for a period of time; although there are certain compositions that are known to be self-nucleating and thus do not require the development of nuclei. Thereafter, the temperature will be raised to temperatures where crystals can grow from the nuclei. The resulting crystals are typically uniformly distributed and fine-grained. Internal nucleation permits glass-ceramics to have favorable qualities such as a very narrow distribution of particle size and a highly uniform dispersion of crystals throughout the glass host.
Glass-ceramics have not been formed by the fusion process because this process requires a much higher viscosity at the liquidus than that available in the precursor glasses of glass-ceramics. Depending upon particular compositions and the forming parameters implemented, the fusion process requires viscosities at the liquidus of at least 75,000 poise, in some cases, of well over 100,000 poises, and more typically above 500,000 poises. The parent glasses of glass-ceramics, designed to crystallize easily, typically have viscosities at their liquidi of 10,000 poises or below, and to our knowledge never above 20,000 poises. They therefore are not amenable to fusion forming. This presents a problem and an opportunity, because glass-ceramics offer desirable properties not achievable in fusion formable glasses. These properties include opacity, various degrees of translucency and surface luster, pastel colors, and perhaps most importantly, a low or essentially zero coefficient of thermal expansion. Thus glass-ceramics have a wider variety of aesthetic appearances as well as heat and fire resistance. Moreover, glass-ceramics are generally stronger than glass, and through surface ion exchange processing can often be made stronger than ion-exchanged glass because of lower stress relaxation at salt-bath temperatures.
Down draw processing of glass, particularly the fusion processing of glass exhibits the inherent advantages of the formation of a resultant pristine surface and the ability to product glass articles (e.g., sheets) exhibiting thinness dimension on the order to 2 mm or less.
As such, it would be desirable to identify glass-ceramics which can be made from fusion formed glasses thus resulting in the formation of thin glass-ceramic articles exhibiting pristine surfaces and exhibiting the intrinsic benefits of glass-ceramics (when compared to glasses), namely strength, low CTE, and associated thermal shock resistance, and color/opacity variation.